1. Field of Invention
The present invention relates to a surface light source device of side light type, a liquid crystal display (LCD) and a guide plate used for the devices, in particular, to the guide plate having an incidence end face, an emission face and back face provided by a minor face and two major faces, respectively, as well as to the devices with improved illumination characteristics or display characteristics by employing the improved guide plate.
2. Related Art
It is well known to employ a surface light source device of side light type for illuminating a LCD panel of a LCD. In general, the surface light source device of side light type comprises a guide plate and a primary light source such as rod-like light source, the primary light source being disposed along an incidence end face of the guide plate. Such arrangement has an advantage that the arrangement can be applied to LCDs without increase in overall size.
Primary light emitted from the primary light source is introduced into the guide plate through the incidence end face. As well known, the guide plate deflects the light through processes such as reflection, scattering or the like, thereby causing the light to be emitted from an emission face which is provided by a major face. Thus emitted light is supplied to the LCD panel through an additional member such as prism sheet.
Two types of guide plates are known. One of the two types is featured by substantially overall uniform thickness and the other is featured by thickness which tends to decrease according to distance from the incidence end face. Generally saying, guide plates of the latter type can emit illumination light more effectively as compared with ones of the former type.
Light scattering pattern is formed on the emission face of the guide plate to adjust brightness distribution by promoting emission therefrom. The light scattering pattern consists of, for example, a great number of light scattering elements arranged according to a certain arrangement rule. The light scattering elements are provided by, for example, a great number of rough local regions formed by processes such as mat-processing or ink deposition.
As well known, the emission face of the guide plate emits light preferentially toward oblique directions inclined forward (namely, as to be away from the incidence end face). This property is called “emission directivity ”.
A prism sheet is disposed along the emission face as a light control member in order to correct this directivity. The prism sheet is an optical member at least one surface of which provides a prismatic surface, being made of light permeable material such as polycarbonate or polyethylene terephthalate. As well known, thus disposed prism sheet causes the emission light to be subject to refraction and inner reflection effect of the prismatic surface, correcting the inclined preferential direction of the emitted light. Such prior art suffers from problem as follows.
If size of the light scattering elements forming light scattering pattern is designed so that brightness is uniform over the emission face, the light scattering elements are hard to be almost invisible to the naked eye. In general, such area as tending to be short of brightness requires light scattering elements formed therein to have a large size. Such large-sized light scattering elements are naturally easy to be visible. Easily visible light scattering elements bring fine bright-dark unevenness to the emission face.
As a result, surface light source devices of side light type employing such guide plates with bright-dark unevenness suffer from reduction in quality (smoothness) of illumination output. In other words, brightness distribution involves rise in high spatial frequency component. And needless to say, LCDs in which such surface light source devices of side light type for backlighting are incorporated suffer from reduction in display quality.
It has been proposed to adjust distribution density of light scattering elements in order to avoid this problem (Japanese Patent Application 228, 852/1997). According the proposed technique, distribution density of light scattering elements is adjusted so as to make emission intensity distribution (brightness distribution) flat under a size condition such that each light scattering element is hard to be visible.
Distribution density is defined as number of light scattering elements allotted and arranged per unit area. Density of light scattering element arrangement is high in an area in which intensive promotion of emission is required while it is low in a portion where weak promotion of emission is required. Such technique realizes improvement in quality of illumination output.
However, the above proposed technique suffers from a unsolved problem. That is, although size of light scattering elements is small, emission promotion property is greatly different between regions (i.e. a set consisting of fine regions) occupied by the light scattering elements and the other region (i.e. a region corresponding to complementary set with respect to the set consisting of the fine regions) without the light scattering elements.
This difference is observed as bright-dark unevenness although it is very fine. It looks some times like a periodic variation of emission intensity which is generated depending on periodicity of the light scattering element arrangement. Such bright-dark unevenness or periodic variation is still a factor of reduction in quality of the emission face's looks or illumination output therefrom, being required to be restrained.
Besides, the above prior arts and proposed technique are apt to be affected by sticking of prism sheet. FIG. 9 is a diagram to give an explain on this problem.
As previously described, a prism sheet is disposed directly on an emission face of a guide plate in many cases. Thus disposed prism sheet tends to partially stick to the emission face. Such sticking produces bright-dark pattern on an output face (i.e. an outer face of the most outside additional member), as shown in FIG. 9, thereby reducing quality of illumination output.
Pattern C like islands and pattern D like dots are exemplarily illustrated in FIG. 9. Pattern C will appear when a rather large local area sticks to the emission face while pattern D will appear when a small local area sticks to the emission face.
Further attention should be paid to another problem which is derivatively generated from such sticking of prism sheet. That is, operation efficiency of assembling of surface light source device of side light type or LCD could be reduced. In particular, when any foreign body such as trash is interposed between a guide plate and a prism sheet, troublesome operations are required for removing the foreign body. If the prism sheet were free from sticking, the foreign body could be removed by rather simple operations.